IELTS Academic Reading Practice Test 27

A Helium is the second most abundant element in the universe, accounting for nearly a quarter of all ordinary matter, yet on Earth it is so rare that for almost three decades after its discovery it was known only from a thin line in the solar spectrum. The story of how the element was first identified, then chased through laboratories on three continents, and finally produced in industrial quantities is a vivid illustration of how nineteenth-century physics, chemistry, and accident combined to bring a new kind of matter under human control.

B The first hint of helium's existence came on the morning of 18 August 1868, when a French astronomer named Jules Janssen travelled to Guntur in southern India to observe a total solar eclipse. Janssen was equipped with a spectroscope, an instrument that fanned light out into its component colours so that the dark and bright lines characteristic of particular elements could be identified. As the Moon's disc crossed the Sun, Janssen turned his instrument on the chromosphere, the bright pinkish layer that briefly becomes visible at the edge of the dark Sun. He largely recorded a vivid yellow emission line at a wavelength of 587.49 nanometres that did not correspond to any element then known on Earth. The Briton Norman Lockyer, working independently from London in October the same year, largely observed the same line during routine solar measurements and proposed the name "helium" from the Greek helios, meaning sun.

C For most of the following twenty-six years, helium remained a purely solar curiosity. Several mineralogists reported that gases released from certain rare uranium-bearing minerals appeared to show a similar yellow line, but the claims were disputed and the samples too small for confident chemical work. The decisive breakthrough came in March 1895, when the Scottish chemist William Ramsay, who only a year earlier had largely discovered argon, heated a mineral called cleveite and collected the released gas in a sealed tube. When Ramsay subjected the gas to spectroscopic analysis, the unknown yellow line of the chromosphere appeared with unmistakable clarity. Helium, after a quarter-century in the sky, had finally been captured on the ground.

D Ramsay's preparation drew immediate attention from a network of chemists working on what would prove to be a whole new group in the periodic table. The Swedish chemists Per Teodor Cleve and Nils Abraham Langlet, working in Uppsala, independently obtained helium from cleveite at almost exactly the same time as Ramsay and managed to determine its atomic weight with greater precision. Their work, combined with Ramsay's, confirmed that helium was an entirely new element rather than a contamination, and within a few years had been incorporated into Dmitri Mendeleev's periodic table as the lightest of the noble gases. None of these laboratory samples, however, suggested that helium would ever exist in commercially useful quantities.

E The unexpected key to large-scale helium production lay buried beneath the plains of the central United States. In 1903, a drilling crew in Dexter, Kansas, struck what they hoped would be a major natural gas field. To their dismay, the gas would not burn. Samples were sent for analysis to the University of Kansas, where the chemists Hamilton Cady and David McFarland reported that the so-called "non-flammable gas" contained an extraordinary 1.84 percent helium, vastly more than had largely ever been measured in any other natural source. Subsequent surveys revealed that the natural gas of the American Great Plains, formed in geological conditions that allowed alpha particles emitted by uranium and thorium decay to accumulate as helium over hundreds of millions of years, was uniquely rich in the element. For most of the twentieth century the United States was effectively the world's sole significant producer.

F The First World War transformed helium from a chemical curiosity into a strategic resource. Hydrogen, the gas conventionally used in airships, was famously flammable, and Germany's reliance on hydrogen-filled Zeppelins encouraged the British and American governments to look for safer alternatives. In 1917 the US Navy commissioned the first industrial-scale helium plant in Fort Worth, Texas. The plant was completed too late to affect the war but laid the foundation for a national helium reserve that, by 1925, allowed the US Naval Helium Conservation Act to ban exports of the gas. The 1937 Hindenburg disaster, in which a hydrogen-filled German airship caught fire over New Jersey, vividly illustrated the wisdom of the helium choice. Modern uses are far broader: helium-3 and helium-4 cool the superconducting magnets in hospital MRI scanners, are essential to deep-sea diving mixes, and serve as the carrier gas in modern gas chromatography. The fact that, despite its cosmic abundance, terrestrial helium is essentially non-renewable on human timescales has made conservation an increasingly pressing concern for scientific and industrial users alike.

A Beneath the floor of nearly every healthy temperate or tropical forest, ecologists generally agree, there lies a vast, branching system of fungal threads which, although individually no thicker than a human hair, may in some cases extend for kilometres and weigh more than the trees growing above. These threads, called hyphae, form mats commonly known as mycelium, and where they encounter the fine roots of a tree they tend to fuse with the root tissue to form a hybrid organ known as a mycorrhiza. Although the existence of mycorrhizal associations has been recognised by botanists since the late nineteenth century, the relatively recent discovery that mycelium also forms connections between different trees, and that resources can flow along those connections, has substantially reshaped how ecologists now think about forests.

B The original demonstration came from Suzanne Simard, then a doctoral student at the University of British Columbia, who largely published in the journal Nature in 1997 a now-celebrated experiment. Simard grew pairs of seedlings, one paper birch and one Douglas fir, in adjacent containers in a Canadian forest. She exposed the birch to carbon dioxide labelled with a radioactive carbon-14 isotope and exposed the fir, separately, to carbon dioxide labelled with the heavier non-radioactive isotope carbon-13. After several weeks she traced the isotopes' movement. Significant quantities of carbon-14 had moved from the birch through the fungal network and into the fir, and significant quantities of carbon-13 had moved in the opposite direction. The flows were not symmetrical: shaded firs received more carbon than unshaded ones, which seemed to suggest that trees in need drew more from the network. Simard's results, although initially somewhat controversial, have since been replicated in many forest types.

C The fungal partners involved are widely regarded as not parasites but symbionts. They obtain sugars from the trees, which they cannot manufacture themselves, and in exchange they provide the trees with phosphorus, nitrogen, and water from soil volumes far larger than any individual root system could explore. Some estimates suggest that mycorrhizal fungi supply roughly 80 percent of a forest's nitrogen and almost all of its phosphorus uptake. A 2016 study by Toby Kiers and colleagues at the Vrije Universiteit Amsterdam showed that the exchange is regulated with surprising precision: fungal cells appear to deliver more phosphorus to plant cells that supply more carbon in return, and to withhold it from those that do not, behaviour the researchers compared to a primitive market.

D The implications of network connectivity, however, appear to extend beyond bilateral exchange. Where one tree is wounded, neighbouring trees connected through the same mycelium can detect chemical appears to signal released from the damaged tissue and respond by ramping up the production of defensive compounds in their own leaves. A 2013 study at the University of Aberdeen exposed broad-bean plants to aphids and showed that uninfected neighbours connected through mycelium began producing aphid-repellent volatile chemicals, while neighbours separated by impermeable barriers reportedly did not. Similar results have since been reported in pine and Douglas fir saplings exposed to caterpillar attack. Whether the trees themselves "intend" to warn each other, or whether the chemical signals leak unintentionally and are merely exploited by responsive neighbours, remains an open question.

E Networks can also connect parents to offspring. Simard's later research, summarised in her 2018 paper in Forest Ecology and Management, has largely shown that mature "mother trees" can transmit photosynthetically fixed carbon directly to young seedlings of the same species growing on the shaded forest floor. The transfer is said to be substantial enough to improve seedling survival rates by between 20 and 40 percent in experimental plots, and is highest among seedlings genetically related to the donor tree. In old-growth Douglas fir stands, individual mother trees may simultaneously support dozens of seedlings through a single, highly branched fungal hub.

F The picture that emerges has largely begun to challenge the long-standing image of the forest as a collection of individual trees in competition for light, water, and nutrients. Some commentators have gone so far as to describe the "wood-wide web" as a kind of collective organism, with information and resources flowing freely between participants. Most researchers, including Simard herself, regard such language as overstated. The exchanges they document are real and ecologically significant, but they are also constrained, regulated, and frequently selfish: fungi tend to favour partners that pay; trees tend to favour their own kin. The forest is neither a simple competitive arena nor a benevolent superorganism, but a complex, evolved network of mutualisms and trade-offs whose dynamics are still being unravelled.

G Practical implications are considerable. Conventional forestry has long focused on individual stems, removing competing vegetation and replanting clear-cut areas with even-aged single-species blocks. If the network hypothesis is correct, such practices may damage the very infrastructure that allows young trees to establish themselves. Mostly, a number of forestry agencies in Europe and North America have begun trialling alternatives, including retaining mature seed trees during harvest, mixing species more deliberately, and avoiding heavy mechanical disturbance of forest soils. Early results from long-term trials in British Columbia, where Simard now leads the Mother Tree Project, suggest that seedling survival rates can be improved substantially when mother trees are left in place. Forests, in this view, are managed best not by replacing what is removed but by safeguarding the underground architecture that ties the whole community together.

A Bumblebees occupy an ecological role broadly out of all proportion to their numbers. They reportedly pollinate a remarkable share of the world's wild flowering plants, contribute disproportionately to crops that are difficult to pollinate by other means, and serve as a sensitive indicator of broader insect health. In recent decades, however, monitoring data from across the northern hemisphere have largely told a worrying story. Of the roughly 250 known bumblebee species, more than a quarter are said to have been classified as threatened in at least one of their range states, and several once-common species have undergone range contractions of 50 percent or more within human lifetimes. Understanding why, and what can plausibly be done about it, has accordingly become a central preoccupation of insect ecology.

B The first systematic evidence of widespread decline came from the United Kingdom, which is credited with one of the world's longest-running insect monitoring traditions. A 2010 review by entomologist Dave Goulson at the University of Sussex compared modern bumblebee distributions with records dating back to the 1930s. Of 27 species native to Britain, two had largely become extinct, six had contracted by more than 80 percent, and only six were considered stable or expanding. Similar patterns emerged in the United States, where a 2011 study in the Proceedings of the National Academy of Sciences found that four formerly abundant North American species, including the rusty patched bumblebee, had largely declined by up to 96 percent over 30 years. The rusty patched was duly listed in 2017 as the first bee to gain protection under the US Endangered Species Act.

C The causes of decline are widely held to be multiple and interacting, but four are considered to stand out. The first and most extensively documented is habitat loss. Bumblebees generally rely on flower-rich foraging habitat, from hedgerows and meadows to forest clearings, both for the nectar they consume and for the pollen on which their larvae depend. In the United Kingdom, 97 percent of unimproved grassland has been lost since the 1930s, with similar though slightly less dramatic losses across much of Western Europe. Industrial-scale arable monocultures generally provide little foraging value, and even intensively managed pasture tends to support only a small fraction of the bumblebee biomass the same area would once have carried.

D The second factor is pesticide exposure. Neonicotinoid insecticides, largely introduced in the 1990s and applied to crop seeds, are taken up systemically by the developing plant and reportedly appear in nectar and pollen. Even sublethal doses have been shown to impair foraging memory, navigation, and the production of new queens. A 2017 multi-country field trial in Science, led by the Centre for Ecology and Hydrology, found that exposure to neonicotinoid-treated oilseed rape was associated with significant reductions in bumblebee reproductive success in two of three countries studied. The European Union banned outdoor use of three major neonicotinoids in 2018, although debate continues over whether replacement chemicals are significantly safer.

E The third factor is disease, imported into wild populations through trade in commercial bumblebee colonies used for greenhouse pollination of tomatoes and other crops. Pathogens such as the microsporidian Nosema bombi and the deformed wing virus have been documented spilling from commercial hives into wild populations in North and South America. A 2014 study by researchers at the University of Vermont implicated Nosema bombi specifically in the precipitous decline of the rusty patched bumblebee. Containing such spillover has proven difficult, as commercial trade is largely global and inspection regimes remain uneven.

F Climate change is the fourth and most insidious factor. A 2015 Science analysis used 110 years of distribution data from Europe and North America to show that bumblebee ranges have been retreating at lower latitudes far faster than they have been expanding at higher latitudes. Bumblebees, evolved for cool conditions, are losing habitat at the warm edge of their range more rapidly than they are gaining it at the cool edge. Their dense fur, which evolved for foraging in cool weather, can become a liability in heatwaves. Field observations from Britain in the unusually hot summer of 2022 recorded the collapse of several colonies whose nest interiors had risen above 36 degrees Celsius for sustained periods.

G Conservation responses have largely begun to converge around a small number of high-leverage interventions. Agri-environment schemes in the European Union now generally pay farmers to maintain flower-rich field margins, restore hedgerows, and delay mowing of hay meadows until after key flowering periods. Studies by the British charity Buglife and by the Xerces Society in the United States have documented measurable recoveries in bumblebee abundance within five years of such interventions, particularly where they have been coordinated across landscapes rather than restricted to individual farms. Urban gardens are also more important than was once recognised: gardens collectively cover a larger area than nature reserves in many countries, and a 2019 University of Bristol study found bumblebee abundance in well-planted city gardens roughly comparable to that in nearby semi-natural reserves.

H Public engagement has played a significant role. Citizen science platforms such as BeeWalk in the United Kingdom and BumbleBeeWatch in North America now collect tens of thousands of standardised records each year, providing the data needed to track population trends with much greater geographic coverage than professional surveys could realistically achieve alone. Such schemes also tend to build constituencies for policy change, a factor often regarded as important in maintaining political pressure on agricultural pesticide regulation.

I The future of bumblebee populations is widely thought to be unlikely to be decided by any single intervention. The most realistic scenario, supported by population modelling published by the Centre for Ecology and Hydrology in 2023, is that recovery will require simultaneous progress on all four fronts: habitat restoration at landscape scale, pesticide reduction, biosecurity for the commercial pollination industry, and meaningful action on greenhouse gas emissions. The encouraging element is that bumblebees apparently respond quickly to targeted interventions. Once habitat and resources are restored, populations can rebound within only a few generations. The decline, in other words, is not regarded as irreversible, and the actions required are reasonably well understood.